1. Field of the Invention
The present invention relates to an image forming apparatus utilizing electrophotography, such as copiers, printers, plotters and printing machines. More particularly, the present invention relates to an image forming apparatus in which an electrostatic latent image is formed on the photoreceptor by irradiating a photoreceptor with a light beam to form a light spot thereon. In addition, the present invention also relates to an electrophotographic image forming method.
2. Discussion of the Background
Various electrophotographic image forming apparatus have been developed and practically used. Electrophotographic image forming apparatus typically include the following processes:
(1) a photoreceptor serving as an image bearing member is charged in a dark place (charging process);
(2) an imagewise light irradiates the charged photoreceptor to selectively decay the charge of the lighted portion of the photoreceptor, resulting in formation of an electrostatic latent image on the photoreceptor (light irradiation process);
(3) the electrostatic latent image is developed with a toner including a colorant such as dyes and pigments and a binder resin such as polymers to from a toner image on the photoreceptor (developing process);
(4) the toner image is transferred onto a receiving material optionally via an intermediate transfer medium (image transfer process);
(5) the toner image formed on the receiving material is fixed upon application of heat and/or pressure thereto (fixing process); and
(6) the surface of the photoreceptor is cleaned with a cleaner after the image transfer process to remove the toner particles remaining on the surface of the photoreceptor (cleaning process).
A photoreceptor used for electrophotography is required to have the following properties:
(1) good charging ability such that the photoreceptor is charged so as to have and maintain a proper electric potential in a dark place;
(2) good charge maintaining ability such that the charges formed thereon hardly decay in a dark place; and
(3) good charge decaying ability such that when the photoreceptor is exposed to imagewise light, the charges of the lighted area rapidly decay and the residual potential thereof is low.
Among these electrophotographic image forming apparatus, digital image forming apparatus in which a laser beam irradiates a photoreceptor to form an electrostatic latent image on the photoreceptor are mainstream now. The digital image forming apparatus are practically used as laser printers, digital copiers and the like apparatus.
The light irradiating process of such digital image forming apparatus typically includes the following sub-processes:
(1) the light output by a laser diode (hereinafter sometimes referred to as a LD) is modulated with digital image data;
(2) the surface of the photoreceptor is raster-scanned with the light beam (i.e., a light spot) emitted from the LD (when a photoreceptor drum is used, the photoreceptor drum is rotated (i.e., the raster-scanning is performed) in a direction perpendicular to the main scanning direction of the light beam), resulting in formation of a dotted electrostatic latent image on the photoreceptor.
In addition, electrophotographic image forming apparatus are currently required to fulfill the following requisites:
(1) to produce high quality images at a high speed;
(2) to be small in size; and
(3) the photoreceptor used thereof has to have a high durability because the photoreceptor has a relatively small diameter compared to conventional photoreceptors.
In general, the life of an electrophotographic image forming apparatus typically depends on the life of the photoreceptor used therefor. This is because the photoreceptor deteriorates relatively seriously compared to other members used for the image forming apparatus since the photoreceptor repeatedly suffers mechanical stresses and chemical stresses in the charging, light irradiating, developing, transferring and cleaning processes.
A photoreceptor is mechanically deteriorated by abrasion and scratches of the surface of the photoreceptor, and is chemically deteriorated by oxidation of the binder resin and the charge transport material included in the photoreceptor due to ozone generated during the charging process and deposition of foreign materials on the surface of the photoreceptor. The mechanical and chemical deterioration of the photoreceptor causes deterioration of image qualities.
As the image forming speed increases and the image forming apparatus is miniaturized, the diameter of the photoreceptor drum is decreased, and thereby the usage conditions of the photoreceptor drum become severer and severer. In particular, in order to well clean the surface of the photoreceptor, a blade made of a hard rubber is used for the cleaner and in addition the contact pressure of the rubber blade with the photoreceptor has to be increased. Therefore, the abrasion of the surface of the photoreceptor is accelerated, resulting in variation of the electric potential and photosensitivity of the photoreceptor. Thereby, problems such that abnormal images are produced; and color balance of produced color images deteriorates, resulting in deterioration of color reproducibility of the color images.
In attempting to improve the abrasion resistance of a photoreceptor, a method in which the photosensitive layer is thickened is proposed and performed. However, when the thickness of a charge transport layer of a multi-layered photosensitive layer, which is typically overlaid on a charge generation layer and which transports the charge generated in the charge generation layer, is increased, the charge moving through the charge transport layer tends to scatter, resulting in increase of the width of electrostatic latent images, and thereby the resolution of the resultant images deteriorates.
In attempting to improve the abrasion resistance of a photoreceptor, a method in which a protective layer is formed on a photosensitive layer or another method in which an inorganic filler is included in a photosensitive layer have been proposed in, for example, published Japanese Patent Applications Nos. 1-205171, 7-333881, 8-15887, 8-123053 and 8-146641. As a result of our experiments, these methods have a drawback in that the area of the photoreceptor lighted by imagewise light gradually increases after repeated use, resulting in deterioration of image qualities such as decrease of the image density, although the abrasion resistance of the photoreceptor can be improved by these methods.
In attempting to remedy the drawback, a protective layer in which a particulate metal oxide is dispersed in a protective layer is proposed in published Japanese Patent Application No. 8-179542.
Although the conventional photoreceptors having a protective layer have good mechanical strength and abrasion resistance but have a drawback in that the resolution of the resultant images deteriorates (the developed toner images widens) due to scattering of the imagewise light in the protective layer.
In addition, it is well know from the above-mentioned background art that in the laser printers and digital copiers in which a laser beam emitted by a LD irradiates a photoreceptor, the particle diameter of the filler included in the protective layer of the photoreceptor is preferably less than the wavelength of the laser light to suppress the scattering of the laser light. However, when the particle diameter of the filler is merely decreased, problems in that the abrasion resistance of the photoreceptor is not improved, and fine line reproducibility of the photoreceptor deteriorates due to diffuse reflection of the laser light on the rough surface of the photoreceptor tend to occur, although scattering of the laser light can be prevented.
Because of these reasons, a need exists for a highly durable electrophotographic image forming apparatus which uses a photoreceptor including a protective layer including a filler and which can produce high quality images for a long period of time without causing deterioration of the image resolution due to scattering or diffuse reflection of the laser light used as the imagewise light.
Accordingly, an object of the present invention is to provide a highly durable electrophotographic image forming apparatus which uses a photoreceptor including a protective layer including a particulate metal oxide filler and which can produce high quality images for a long period of time without causing deterioration of the image resolution due to scattering or diffuse reflection of the laser light used as the imagewise light.
Briefly this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by an image forming apparatus including:
a photoreceptor which serves as an image bearer and which includes an electroconductive substrate, a photosensitive layer including a charge generation material and a charge transport material and located overlying the electroconductive substrate, and a protective layer including an inorganic filler having an average particle diameter (d) and a binder resin;
an imagewise light irradiating device configured to irradiate the photoreceptor with a laser light beam having a wavelength of (xcex) while scanning the laser light beam to form light spots each having a diameter (L) in the minor axis direction thereof on the surface of the photoreceptor and to form a latent image on the photoreceptor,
wherein the following relationship is satisfied:
0.1 less than 3.75xc3x9710xe2x88x923L/xcex less than d/xcex less than 0.5. 
The inorganic filler included in the protective layer preferably has an average particle diameter of from 0.2 to 0.4 xcexcm.
The diameter (L) of the light spot in the minor axis direction is preferably from 10 to 80 xcexcm.
It is preferable that the protective layer further includes a charge transport material.
The photosensitive layer preferably is a multi-layered photosensitive layer in which a charge generation layer including the charge generation material and a charge transport layer including the charge transport material are overlaid.
The filler included in the protective layer is preferably a material selected from the group consisting of titanium oxide, silica, alumina and mixtures thereof.
The wavelength of the laser light beam is preferably a wavelength of from 400 to 450 nm.
The image forming apparatus can include a process cartridge including the photoreceptor and at least one of a charger configured to charge the photoreceptor; an image developer configured to develop the electrostatic latent image with a developer including a toner to form a toner image on the photoreceptor; and a cleaner configured to clean the surface of the photoreceptor (i.e., to remove the residual toner from the surface of the photoreceptor).
In the another aspect of the present invention, an image forming method is provided which includes the steps of:
irradiating a surface of photoreceptor with a laser light beam having a wavelength of (xcex) to form a light spot having a diameter (L) in the minor axis direction thereof on the surface of the photoreceptor,
wherein the photoreceptor includes an electroconductive substrate, a photosensitive layer including a charge generation material and a charge transport material and located overlying the electroconductive substrate, and a protective layer comprising an inorganic filler having an average particle diameter (d) and a binder resin, and
wherein the following relationship is satisfied:
0.1 less than 3.75xc3x9710xe2x88x923L/xcex less than d/xcex less than 0.5. 
In the present application, the diameter of a spot of a laser beam is defined as follows. The light intensity of a laser beam spot has a Gaussian distribution. The diameter of a light spot is defined as a diameter of a circle (or an ellipse) at which the light intensity of the laser light is 1/e2 of the maximum light intensity of the laser beam spot, wherein e represents Euler""s constant (i.e., 2.718). When the light spot has an ellipse form, the minor axis diameter of the ellipse is defined as the diameter of the light spot.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.